The present invention relates to a method for producing an embossed poly(vinyl alcohol) (hereinafter to be referred to as PVA) film. More particularly, this invention relates to a method for producing an embossed PVA film superior in appearance, resistance to blocking and resistance to pinholes.
Conventionally, a packaging material using a PVA film has been widely used as a material for containing agricultural chemicals, medicines, dyes, detergents, fertilizers, cosmetics, sanitary products and the like. This packaging material is used for, for example, obtaining a solution of what is included in the container utilizing the water solubility of PVA, by directly casting the container into an aqueous medium to allow disintegration, dispersion or dissolution of the container, for the disposal of the packaging material opened when in use, by dumping the packaging material into water as it is, and for other purposes. However, because the PVA film absorbs moisture, the surface sometimes becomes sticky and tends to show blocking of film due to the adhesive property of the film in high humidity.
To prevent the blocking property, films are generally embossed. For example, an embossed water soluble film has been proposed, which has a thickness of a concave part of 15-25 xcexcm, and a tensile strength of the film of 2.5-4.5 kg/mm2 (JP-A-7-290567).
While the above-mentioned film shows improved dispersibility and dissolution in water, mechanical strength and the like, it is otherwise with regard to resistance to blocking, appearance to impart an impression of high quality, resistance to pinholes and the like and these aspects have not been considered sufficiently.
As to the resistance to blocking, it is a general practice to evaluate same by applying a pressure to a film and measuring the blocking property of the film. When the film is in an actual mode of use, namely, when the film has been wound in a roll when the evaluation of the blocking property is performed, the evaluation becomes closer to an actual level evaluation and requires severer resistance to blocking than it does for the evaluation of the above-mentioned film.
As to the resistance to pinholes, conventional embossing is done by passing a film between an emboss roll and a backup roll. An embossed film is subject to the occurrence of pinholes. When a paste or liquid substance is contained, it may ooze out or leak out. Thus, a further improvement is desired.
It is therefore an object of the present invention to provide a method for producing an embossed PVA film superior in appearance and resistance to pinholes, which is capable of improving resistance to blocking and retaining resistance to blocking even if stored in a roll state.
The present invention therefore provides a method for producing an embossed PVA film, which comprises embossing a PVA film in such a manner that makes a crystallinity index at the emboss top fall within the range of 0.50-0.90.
Furthermore, when the ratio of the apparent film thickness (T xcexcm) after embossing to the film thickness (t xcexcm) before embossing (T/t; hereinafter sometimes to be referred to as degree of embossing) is 1.5-5, the action and effect of the present invention become more striking.
When the above-mentioned embossed PVA film is produced, a PVA resin having a viscosity in a 4 wt % aqueous solution of 5-40 mPaxc2x7s (20xc2x0 C.) and a saponification degree of 70-99 mol % is used, or a PVA film is passed between an emboss roll and a backup roll for embossing, or a backup roll satisfying the conditions of a surface hardness (as measured by the method according to JIS K 6301) of A50-A95 and/or a surface hardness (as measured by the method according to JIS K 6253) of D60-D90 is used, or a moisture content of the PVA film is adjusted to 4-15 wt %, hereby the objective film can be preferably produced.